Dimensions, drill sizes and wrench sizes for 1-1/2-18 UNEF.
Designation
1-1/2-18 UNEF
Major (outside) diameter
38.1 mm · 1.5"
Pitch
1.411 mm · 18 TPI
Pitch diameter
37.183 mm · 1.4639"
Minor diameter (external)
36.369 mm · 1.4318"
Minor diameter (internal)
36.572 mm · 1.4399"
Recommended tap drill
1-29/64" · 36.91 mm · ≈65% thread
Clearance hole
Close 1-33/64" · Normal 1-17/32" · Loose 1-9/16" - nominal + 1/64" / 1/32" / 1/16" (common shop practice), given as the sums themselves because they are larger than the 1" end of the standard drill table.
Hex head (across flats)
2-1/4"
Close 1-33/64" · Normal 1-17/32" · Loose 1-9/16" - nominal + 1/64" / 1/32" / 1/16" (common shop practice), given as the sums themselves because they are larger than the 1" end of the standard drill table.
1-1/2-18 UNEF in numbers
1-1/2-18 UNEF is 1.5000 in across the crests with 18 threads per inch, a 1.411 mm pitch. The basic pitch diameter is 37.183 mm and the bolt's minor diameter 36.369 mm, so the thread is 0.866 mm deep on the bolt. Its tensile stress area is 1.6419 in², 17% more than the coarser 1-1/2-6 UNC at the same diameter. The nearest metric size, M39 × 4.0, is 0.90 mm larger across the crests and its pitch is 2.589 mm coarser, so the two do not interchange.
Tap drill by thread percentage
Hole = D − % × 0.0129904 × P, with D = 38.1 mm and P = 1.411 mm. A smaller hole leaves more thread; the recommended 1-29/64" drill above gives about 65%.
Tap-drill hole diameters for 1-1/2-18 UNEF at several thread percentages.
Thread
Hole (mm)
Hole (in)
60%
37.00
1.4567
65%
36.91
1.4531
70%
36.82
1.4495
75%
36.73
1.4459
Tightening torque estimate
Target preload is 75% of proof load; torque is estimated from T = K · F · d.
Proof-load preload and estimated tightening torque for 1-1/2-18 UNEF by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
1.6419 in²
40637 lbf
1016 lb·ft · 12191 lb·in
SAE Grade 5
1.6419 in²
91126 lbf
2278 lb·ft · 27338 lb·in
SAE Grade 8
1.6419 in²
147772 lbf
3694 lb·ft · 44332 lb·in
Strengths are SAE J429 for this diameter band. J429 steps down with size: Grade 2 falls to 33 ksi proof above 3/4 in and Grade 5 to 74 ksi above 1 in, so these figures are not the small-size values.
These figures assume a nut factor of K = 0.20 (dry / as-received steel). K captures thread and head friction and can swing torque by 30% or more - a lubricated joint needs noticeably less. Change the lubrication assumption in the bolt-torque calculator.
Put 1-1/2-18 UNEF on a job sheet
The bolt schedule collects the fasteners for one job onto a single sheet - tap drill, wrench size, grade and tightening torque on every line, taken straight from pages like this one. Free schedules hold 3 lines and are saved in this browser only.
For a 1-1/2-18 UNEF thread, drill the hole with a 1-29/64" tap drill (36.91 mm) before tapping. That produces roughly 65% thread engagement, the usual target for a strong, tappable hole. The mating bolt has a major diameter of 38.1 mm and a pitch of 1.411 mm (18 TPI).
To pass a 1-1/2-18 UNEF bolt through a part rather than thread it, use a clearance hole: Close 1-33/64" · Normal 1-17/32" · Loose 1-9/16" - nominal + 1/64" / 1/32" / 1/16" (common shop practice), given as the sums themselves because they are larger than the 1" end of the standard drill table..
All values come from the standard thread formulas (ISO 68-1 / ASME B1.1) and published standard tables. See the full reference.
Frequently asked questions
What drill size do I need to tap a 1-1/2-18 UNEF thread?
Drill 1-29/64" (36.91 mm) before tapping 1-1/2-18 UNEF. That leaves roughly 65% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 1-1/2-18 UNEF a coarse or fine thread?
1-1/2-18 UNEF is a Unified extra-fine (UNEF) thread at 18 threads per inch, used for thin-wall parts, short thread engagement and fine adjustment.
What size clearance hole does a 1-1/2-18 UNEF bolt need?
Close 1-33/64" · Normal 1-17/32" · Loose 1-9/16" - nominal + 1/64" / 1/32" / 1/16" (common shop practice), given as the sums themselves because they are larger than the 1" end of the standard drill table. A clearance hole lets the 1-1/2-18 UNEF bolt pass through the part instead of threading into it.
How much torque should I use on a 1-1/2-18 UNEF bolt?
Estimated tightening torque for 1-1/2-18 UNEF runs from 1016 lb·ft (SAE Grade 2) to 3694 lb·ft (SAE Grade 8), targeting 75% of proof load with a dry nut factor of K = 0.20. Lubricated threads need noticeably less torque.
Nearest standard metric sizes by major diameter. These are substitutes by size only - the thread forms are not interchangeable, so never run one into the other.